Clamping devices with a wedging action are known in many different constructions. These constructions fall into three basic types. An open type, a closed type, and a semi-open type are recognized. Such clamping devices are also classified in accordance with the intended type of stress to be applied to a test sample, namely for applying tension stress or for applying compression stress, or for applying tension for compression stress as desired. The construction will differ for the type of stress to be applied to the test sample.
German Patent Publication DE-AS 2,141,566, for example, discloses a clamping device with a wedging action of the open type construction. Such a structure is used exclusively for testing samples with regard to tensile stress, whereby the samples must have either a circular or a rectangular flat cross-sectional configuration. This known clamping device with a wedging action, has a clamping housing with a wedge-shaped clamping mouth formed by respective wedge-shaped clamping elements referred to as wedging clamping jaws mounted in said clamping housing for an axial displacement. Due to the wedge shape of the clamping mouth and of the clamping jaws, the latter generate simultaneously with an axial movement a radially directed clamping movement. In order to apply the clamping action to a test sample, the clamping jaws are operated by a piston cylinder device in an axial direction. In order to again release a test sample from the clamping jaws, the piston cylinder device is depressurized. Tension springs are connected to the clamping jaws in such a way that they counteract the clamping force of the piston cylinder device, whereby the clamping jaws upon depressurizing the piston cylinder device, are returned by the tension springs into the starting position, thereby to release the clamping action.
It is also known from European Patent Publication No.: 0,257,104 to provide a piston cylinder device for opening the clamping jaws by causing a retraction movement of these clamping jaws. In such a structure the clamping housing is constructed so that the sample to be tested can be inserted into the clamping housing in a lateral direction. This feature speeds up and simplifies the clamping operation of the sample. The clamping jaws and the clamping housing in which these jaws are mounted, are so constructed that a self-locking feature of the wedges of the clamping jaws can be utilized by applying a tension force to the sample.
The just described so-called open type of construction of the wedging clamping device has the disadvantage that due to the forces that the clamping housing itself must take up when a sample is loaded, the clamping housing must have relatively large dimensions so as to avoid any elastic reaction of the clamping housing which may result in the release of the test sample. For example, German Utility Model Publication 1,977,205discloses a structure intended to minimize the elastic reaction of the clamping housing. In order to strengthen the clamping housing, a ring-shaped clamping piece is arranged around the clamping housing and the ring-shaped clamping piece is operable by a hydraulic drive.
U.S. Pat. No. 3,335,603 (Gram) discloses a wedge-type clamping device of the so-called closed construction capable of applying tension loads or compression loads to a test sample. This known clamping device comprises clamping wedges arranged in a housing closed on all sides, except for an axial inlet opening for permitting the insertion of the test sample end into the clamping device. Thus, it is a disadvantage of the closed type of construction that the sample must be inserted in the axial longitudinal direction which requires the axial displacement of at least one of the two clamping devices or clamping heads needed for holding a test sample. Such axial displacement is time consuming and requires additional structural features for performing the axial displacement. However, the closed type of construction has the advantage that it has a small overall structural height due to the closed type construction of the housing.
Another known clamping device is constructed particularly for applying a compression load to a sample. The construction is such that the compression force introduced into the test sample causes a closing of the wedges for applying the clamping action to the sample.